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https://doi.org/10.1038/s43247-022-00653-w
OPEN
Nucleation of jet engine oil vapours is a large
source of aviation-related ultrafine particles
1234567890():,;
Florian Ungeheuer 1, Lucía Caudillo1, Florian Ditas 2, Mario Simon1, Dominik van Pinxteren3,
Doğuşhan Kılıç4,5, Diana Rose2, Stefan Jacobi2, Andreas Kürten1, Joachim Curtius 1 & Alexander L. Vogel
1✉
Large airports are a major source of ultrafine particles, which spread across densely populated residential areas, affecting air quality and human health. Jet engine lubrication oils are
detectable in aviation-related ultrafine particles, however, their role in particle formation and
growth remains unclear. Here we show the volatility and new-particle-formation ability of a
common synthetic jet oil, and the quantified oil fraction in ambient ultrafine particles
downwind of Frankfurt International Airport, Germany. We find that the oil mass fraction is
largest in the smallest particles (10-18 nm) with 21% on average. Combining ambient
particle-phase concentration and volatility of the jet oil compounds, we determine a lowerlimit saturation ratio larger than 1 × 105 for ultra-low volatility organic compounds. This
indicates that the oil is an efficient nucleation agent. Our results demonstrate that jet oil
nucleation is an important mechanism that can explain the abundant observations of high
number concentrations of non-refractory ultrafine particles near airports.
1 Institute for Atmospheric and Environmental Sciences, Goethe-University Frankfurt, Frankfurt am Main 60438, Germany. 2 Department for Ambient Air
Quality, Hessian Agency for Nature Conservation, Environment and Geology, Wiesbaden 65203, Germany. 3 Atmospheric Chemistry Department (ACD),
Leibniz Institute for Tropospheric Research (TROPOS), Leipzig 04318, Germany. 4 The Department of Earth and Environmental Sciences, the Faculty of
Science and Engineering, the University of Manchester, Manchester M13 9PS, UK. 5 National Centre for Atmospheric Science, Manchester M13 9PS, UK.
✉email: vogel@iau.uni-frankfurt.de
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S
everal studies identified airports as a major source of
ultrafine particles (UFPs)1–8. Among different engine
operation conditions at the airports, take-off is often associated with the highest UFP emissions3,9–12. These particles are
typically formed via gas-to-particle conversion after
combustion13. Transmission electron microscopy analysis of
UFPs from aviation shows spherical particles with a volatile
character under high vacuum14. They can be transported large
distances from the source reaching densely populated residential
areas, as large airports are usually located in the close vicinity of
metropolitan areas3,15,16. UFP emissions from airport operations
lead to a higher ambient particle number concentration (PNC) in
the surrounding of airports, with a limited knowledge of their
chemical composition17,18. UFP transport and subsequent infiltration to the indoor environment seems to be more relevant than
infiltration of PM2.5 and PM1018,19. The number-size distributions of particles emitted by jet engines are dominated by a mode
diameter smaller than ~30 nm, which is significantly smaller
compared to particles from road traffic emissions12,20–23. Jet
engine oil constituents (Supplementary Fig. 1) have been identified in UFPs near airports2,7,24–26. Lubrication oils are emitted
from aircraft engines through a breather vent and unintentionally
as leaks of the oil circulating system (i.e., due to worn seals)24.
Due to the small size of UFPs, exposure-related health effects are
of importance as they potentially reach the alveoli, penetrate
Fig. 1 Particle size distributions of ambient and laboratory-generated
ultrafine particles. Ambient particle size distribution (a) at the monitoring
site during wind direction (WD) from the airport (grey) and the city
(yellow), averaged over three days (05:00–23:00 CET). Number-size (b)
and mass-size distribution (c) from two laboratory experiments, each with
jet oil nanoparticles generated from a methanolic solution, native at 20 °C
(blue & light blue) and after heating to 300 °C (red & light red).
Measurement data (black dots) were fitted using a lognormal distribution.
2
through the pulmonary epithelium in the lower respiratory tract,
and translocate the air-blood barrier27–30. Animal tests also showed
that they can reach the central nervous system via the olfactory
nerve circumventing the blood-brain barrier31. UFPs can permeate
into the respiratory and cardiovascular system within minutes to
hours and are still detectable for months after the exposure32.
Depending on their chemical composition, UFPs can induce oxidative stress, inflammatory reactions, and cell membrane
damages33–35. Health effects depend on their particle size, mass and
number concentration5, and additionally on individual properties
such as surface area, solubility, oxidative potential and the ability to
counteract macrophage phagocytosis36. Several studies investigated
the UFP exposure of airport ground personnel and passengers37,38
and health effects due to UFP exposure near the airport6. A recent
cohort study reported a 12% increased risk of developing a malignant brain tumour in the Los Angeles airport area for each increase
of UFP exposure by 6,700 particles cm−3 39. This finding is supported by a study from Toronto, which reports a hazard ratio of
1.112 in developing a malignant brain tumour per UFP increase of
10,000 particles cm−3, adjusted for other air pollutants and sociodemographic factors40. A study of the health effects from long-term
UFP exposure of airport workers reported no association to cardiovascular disease41.
Ultrafine and fine particle emissions by jet engines during flight
have also been investigated42,43. Here the focus has been put on
determining emission indices for particle emissions at cruise and
their role for contrail and cirrus formation44. Black carbon (soot)
emissions have been discussed to dominate the formation of ice
crystals in contrails, especially in the soot-rich regime characterised
by soot particle number emission indices, EIs, in excess of ~1014 (kgfuel)−1 44. Recent studies have shown that soot formation by aircraft
engines burning plant-based bio-fuels blended with petroleum-based
conventional kerosene (Jet A) or blends of synthetic fuels (FischerTropsch) with Jet A fuel, both significantly reduces the soot
formation43,45, which is likely explained by the near zero aromatic
contents of the bio and synthetic fuels. Ultrafine volatile particles
were assumed to be mostly composed of sulphuric acid and organic
fuel components that nucleate in the young exhaust plume46,47, but
jet lubrication oil has so far not been suggested as an important
source of the freshly formed particles in the exhaust plume in flight.
In our previous study on airport-related UFPs, we showed that
jet engine lubrication oils dominate the spectrum of detected
organic compounds after a non-target analysis26. Following this
non-target study, here we describe the nucleation ability of jet oil
vapours in laboratory experiments and by quantification of jet
engine oil constituents in three ambient UFP size fractions
(<56 nm) downwind Frankfurt International Airport.
Results
Volatility and new-particle formation of jet engine lubrication
oil. We compared particle-number size distributions (PNSD) of
ambient UFPs with laboratory-generated jet oil particles. In the
ambient measurements at Frankfurt-Schwanheim (Supplementary
Fig. 2) we observe a distinct difference between UFPs from the
airport and the city centre (Fig. 1a). Air masses transported from
Frankfurt Airport show a ~15-times higher PNC of UFPs at
~18 nm compared to air masses from the city centre (wind roses are
shown in Supplementary Fig. 3). For larger UFPs, this difference
becomes less pronounced. In the laboratory, we studied the PNSD
of atomised lubrication oil passing a thermodenuder at 20 °C and
300 °C to investigate the volatility and nucleation capability of the
jet oil compounds. When the jet oil particles (mean diameter of
27 nm) pass the thermodenuder at 300 °C, we observe a more than
fivefold increase of the particle number concentration compared to
the experiment at 20 °C, and a reduction of the mean diameter
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down to ~10 nm of the measured particles (Fig. 1b). Although the
particles passed the thermodenuder, it is important to mention that
the PNSD measurement was conducted downstream the heating
section at room temperature. The volatility of the jet oil at 300 °C is
evident as the mass fraction of jet oil is reduced by ~99% compared
to the 20 °C control experiment (Fig. 1c). Downstream the heating
section of the thermodenuder the majority of oil vapours in the gas
phase is likely lost to the surfaces of the tubing. However, a small
fraction of the oil vapours nucleates and forms new particles
downstream of the thermodenuder within a few seconds, when the
temperature of the sampling flow reaches a point at which the oil
vapour becomes supersaturated. Rapid growth of particles to sizes
>10 nm allows escaping the “valley of death” in the nucleation
mode48, in which small particles are efficiently scavenged by coagulation. The thermodenuder experiment demonstrates that jet
engine oil particles are volatile UFPs at 300 °C, and it can be
assumed that the oil partitions entirely to the gas phase if exposed to
operating temperatures of aircraft turbofan engines (»300 °C49).
Fraction of lubrication oil in ambient UFP samples. We
quantified jet engine oil constituents (Supplementary Fig. 1 and
Supplementary Note 1) from ambient particle samples to determine the oil fraction in UFPs near Frankfurt Airport. Therefore,
we collected UFP samples downwind the airport at FrankfurtSchwanheim when air masses arrived from the airport (Fig. 2a–d).
Using a cascade impactor (Nano-MOUDI), we sampled UFPs
during seven periods (18–54 h) in three different UFP size bins
(10–18 nm, 18–32 nm, 32–56 nm) for subsequent chemical analysis. From the continuous measurements of the PNSD we calculated the mass concentration (oil density = 1 g cm-3, see
Durdina et al.50) for the three investigated particle size bins
(Fig. 2b–d). The corresponding UFP number concentration is
shown in Supplementary Fig. 4. Particle mass concentration of the
two smallest size bins (<32 nm) increased significantly (two-tailed
t-test, p < 0.001) when the wind direction falls within the airport
sector during its operating hours, compared to periods of other
wind directions or non-operating hours. The variability of larger
UFPs (>32 nm) does not show this behaviour (Fig. 2d–g). This is
in accordance with previous studies, which state that the mode
diameter of aircraft-related particle emissions is smaller than
30 nm, while the mode diameter of particles from on-road vehicles
is predominantly larger than 30 nm3,12,22,23. The particle number
concentration (<32 nm) reaches the rural background level
around midnight. Hence, we consider the night-time periods
between 00:00–05:00 CET adjacent to each sampling day as the
mean rural background particle mass concentration that is largely
unaffected by UFPs from the airport (dark red bars in Fig. 2b–d).
Subtraction of the mean background mass from the mass during
UFP sampling results in the total accumulated UFP mass on each
impactor stage that can be attributed to the airport (Supplementary Figure 5 & Supplementary Table 1). This approach of mass
closure cannot be applied to the largest stage (32–56 nm), because
the particle mass concentration reaches sometimes higher values
during non-operating than during operating hours (Fig. 2d).
We quantified the jet oil concentration of the individual
impactor stages by adding authentic standards to aliquots of the
filter extracts (standard addition method). Furthermore, we
corrected for particle losses in the Nano-MOUDI based on an
experimentally determined loss function of the three nano-stages
(see Methods section). We find that jet engine oils contribute on
average 21 ± 11% to the UFP mass in the 10–18 nm size bin. The
jet-oil mass fraction of individual samples in the 10–18 nm size
bin varies between 10 and 38%, with generally higher values for
short sampling intervals. The contribution of jet engine oil to the
total mass of the 18–32 nm particles is only 5 ± 3% on average
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(Error estimation see Supplementary Note 2). Because the
background subtraction could not be applied on the largest stage,
we used the non-background corrected SMPS mass of the
32–56 nm stage and find a mean of 9% for the oil fraction of this
size bin. Hence, the smallest particle stage shows consistently the
highest mass fraction of jet engine lubrications oils (Supplementary Table 1 and Supplementary Fig. 6).
The calculation of the fractional oil contribution on all three
stages did take into account experimentally determined particle
losses. Regarding evaporative losses, we observe a bias in the
molecular composition of jet oil from the ambient samples, which
can be well explained by evaporation of the semi-volatile additives
during Nano-MOUDI sampling (Supplementary Table 2). We
evaluated the sampling efficiency of the Nano-MOUDI toward
semi-volatiles based on pure ethyl oleate UFPs (C20H38O2, 98%,
Sigma-Aldrich) from an atomised solution. Although the
generated PNSD covered the whole Nano-MOUDI range, we
only detected the compound on the 32–56 nm stage (with the
lowest pressure difference of the three Nano-MOUDI stages), and
even on this stage we observed a loss of >99% of mass of
the ethyl-oleate-UFPs. The vapour pressure of ethyl oleate is
8.10 × 10-3 Pa (EPI Suite51), which is similar to the vapour
pressure of the N-phenyl-1-naphthylamine jet oil additive. The
other additives and the jet oil esters exhibit lower vapour
pressures (Supplementary Table 2). Therefore, it can be stated
that the vapour pressure, and with this regard the volatilisation of
semi-volatile compounds is the most important sampling loss
process in the Nano-MOUDI. Fortunately, the jet oil esters are
extremely low-volatile, and therefore evaporation of this compound class during sampling is negligible.
Lubrication oil base stock esters in the volatility basis set. The
observed new-particle formation downstream the thermodenuder and the largest mass fraction of lubrication oil in the
smallest ambient UFPs suggests that lubrication oil emissions
from jet engines play a pivotal role in nucleation and early
growth of new particles. We further evaluated this hypothesis
by classifying the oils’ synthetic esters into the volatility basis set
(VBS52–54). Figure 3a shows the quantified ambient particlephase concentration of single esters from two different jet oil
base stocks: pentaerythritol esters (C27-38H48-70O8) and trimethylolpropane esters (C27-34H50-64O6). We used the SIMPOL.1 model55 to estimate the vapour pressures of the different
esters. We then calculated their saturation mass concentration
C i (at 293.15 K), which is the inverse of the gas-to-particle
partitioning constant [Eq. 1], and assigned them to volatility
classes56. In the ambient UFP samples, we measured particlephase concentrations of the esters between 0.01 and 4 ng m−3.
Following, we calculated the theoretical gas-phase concentration, assuming that the esters’ partitioning would shift entirely
to the gas phase at ambient temperatures [Eq. 2]. As the
lubrication oil concentration is not corrected for atmospheric
dilution between the airport and the measurement station at
Frankfurt-Schwanheim, the gas-phase concentration in the
engine exhaust plumes at high temperatures (>300 °C) is certainly higher than this lower-limit estimate downwind of the
airport. However, we still observe a large saturation ratio of the
theoretical gas-phase concentration [Eq. 3], which we derived
from ambient particle-phase concentrations (Fig. 3b). The three
largest pentaerythritol esters, which fall into the region of ultralow volatility, reach a saturation ratio of up to 3 × 105. Although
this calculation is a lower-limit estimate, it supports the
hypothesis that the synthetic esters from lubrication oils can
initiate rapid nucleation in the exhaust plume of aircraft
engines. Based on the theoretical gas-phase concentration, we
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Fig. 2 Overview of the UFP mass concentrations derived from PNSD measurements and wind direction at Frankfurt Airport. Wind direction (a) with
wind speed indicated by the colour code. The wind data is provided by the meteorological station at Frankfurt Airport (International Civil Aviation
Organization, ICAO, code: EDDF) of the German weather service (DWD). The ambient UFP mass concentration (µg m-3) in the size ranges 10–18 nm (b),
18–32 nm (c) and 32–56 nm (d), and the Nano-MOUDI sampling intervals (horizontal lines), indicating the average mass concentration during sampling
hours (blue) and the average background mass concentration (dark red). Boxplots in (e) show the spread of the total mass concentrations (µg m-3) for
10–18 nm, 18–32 nm and 32–56 nm particles during airport operating- and non-operating hours (based on SMPS data of the sampling- and background
correction periods). The bottom and top of the boxes indicate the interquartile range and the horizontal line inside the boxes indicates the median. The
whiskers show the scatter towards the most extreme values. The wind roses depict the prevailing wind directions during filter sampling at airport operating
hours (f). Furthermore, the wind directions during the non-operating hour periods used for SMPS background correction are shown (g).
also determined the temperature at which each single ester
compound reaches gas-phase supersaturation ðSi >1Þ during
cool-down of the exhaust plume [Eq. 6] (Fig. 3c). At ~60 °C, the
ultra-low volatility pentaerythritol esters (C36H66O8–C38H70O8)
are the first compounds that reach Si >1, although their ambient
concentration is an order of magnitude lower than the
extremely-low volatility ester C29H52O8. Based on our measurements we observe that all synthetic esters reach
4
supersaturation at ambient temperature, except the three most
volatile trimethylolpropane esters C27-29H50-54O6.
Discussion
We interpret our laboratory thermodenuder experiment in such
way that heated oil particles from an atomised solution generate
gaseous oil vapours, which nucleate and form new ~10 nm
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Fig. 3 Ambient concentrations of jet oil esters in the volatility basis set.
a Quantified ambient particle concentration (ng m−3) of each synthetic
ester compound (C27-38H48-70O8 (red boxes); C27-34H50-64O6 (blue
boxes)) plotted against the log10 Ci at 20 °C (ULVOC: ultra-low volatility-,
ELVOC: extremely-low volatility-, LVOC: low-volatility organic compound).
The boxes show the spread of the quantified mass concentrations
(10–56 nm) as interquartile range during airport operating hours. The
horizontal line within the boxes shows the median and the whiskers show
the spread to the most extreme values. Values outside ±2.7 σ are marked as
outliers by “o” symbols. b Resulting gas-phase saturation ratio Si (20 °C) of
the theoretical gas-phase concentration when all particle-phase compounds
of (a) would partition to the gas phase. Values of Si (20 °C) above 1
(yellow line) indicate supersaturation. c The approximate temperature at
which the different jet oil esters reach Si ¼1.
particles right behind the heated section. This experiment
demonstrates that the jet oil compounds are volatile at 300 °C, but
also efficient nucleators at room or ambient temperature. The
particle diameter of the freshly nucleated particles in our
laboratory experiment appears in the same size region as the
ambient UFPs downwind of Frankfurt Airport. Certainly, these
laboratory experiments do not reflect the full complexity of jet
engine emissions in the atmosphere. In real emission plumes,
non-volatile particulate matter (nvPM) could scavenge nucleation
by providing surface for condensation of oil vapours. However,
earlier studies describe aviation-related UFPs as volatile under
high vacuum14, therefore, it appears likely that a large number of
these particles, which are observed downwind of airports12,23, are
formed via nucleation of gaseous jet oil emissions.
Efficient nucleation and growth by organic compounds
requires both (ultra-low) volatility compounds and sufficient high
gas-phase concentrations for growing the particles fast enough.
The lower concentration of the three ultra-low volatility organic
compounds (ULVOCs) and higher concentrations of extremelylow volatility organic compounds (ELVOCs) create ideal conditions for initial nucleation by the jet oil ULVOCs, followed by
rapid growth due to condensation from a large gas-phase reservoir of jet oil ELVOCs. The range of critical temperatures at
which the compounds reach supersaturation suggests that
nucleation and particle growth occurs in the near-field during
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cool down of hot exhaust behind the turbofan (Fig. 4), and can
explain the large volatile fraction (at elevated temperatures) of
UFPs from aviation. To which extent the emission of nvPM from
turbofan engines can scavenge this nucleation and growth needs
further evaluation.
In fact, the reduction of nvPM emissions (e.g. soot) from aircraft engines in the last decades57—and with this the reduction of
the condensational sink—might have led to an increase of the
number concentration of volatile UFPs that are formed via
nucleation of gaseous oil vapours or sulphuric acid. Nevertheless,
the determined high gas-phase saturation ratios of the ULVOC
synthetic esters suggest that nucleation can occur despite the
presence of the condensation sink from nvPM. The dynamics of
formation and condensation of semi-volatile oxidation products
(e.g. from incomplete combustion) are not investigated in this
study, but are complementary for understanding the UFP composition. Hence, the above-mentioned Nano-MOUDI sampling
artefacts are critical, as low- and semi-volatile oxidation products,
which can contribute to UFP mass, are lost during sampling.
Despite these uncertainties and considering that our results are
lower-limit estimates, they substantiate the main finding that jetoil vapours reach gas-phase supersaturation in cooling emission
plumes leading to rapid nucleation and formation of UFPs in the
range of ~10–20 nm.
Our observations of lubrication oil emissions being an
important source for UFPs implies that this source will not be
addressed by replacing traditional jet fuels with sustainable
aviation fuels (SAF)45, and should therefore also be taken into
account in the current endeavour to eliminate UFP emissions
from aviation. Accordingly, the air/oil separator should be optimised with regard to an improved jet oil recovery, and thus
preventing oil emissions. In addition, developing advanced
maintenance routines and reducing the total uptime of jet engines
at airports (e.g. through electrification of ground handling) could
also reduce oil emissions. Furthermore, evaluation of the toxicological properties of jet oil UFPs should be conducted to assess
their health effects, also considering detrimental and potentially
neurotoxic substances that are either directly emitted (e.g. organophosphates as lubrication oil additives58,59), or which are
formed through thermal transformation of the utilised trimethylolpropane esters (e.g. trimethlyolpropane phosphate)26,60.
Furthermore, lubricant oil emissions during cruise and their
possible effects on cirrus cloud formation needs further investigation, as the oil effect (e.g. as an organic coating on soot particles) has not been studied, yet.
Materials and methods
Jet engine oil thermodenuder measurements. We used thermodenuder measurements to determine the volatility of jet oil UFPs, as the new international
aircraft particulate matter standard only considers the number and mass concentration of nvPM61. We determined the particle-number size distribution of
Mobil JetTM Oil II UFPs, formed using an atomizer (replica of TSI model 3076)
with 0.04 g L−1 jet oil solved in ultra-pure methanol. The resulting PNSD downstream of the thermodenuder (operated at 20 °C and 300 °C) was measured using a
scanning-mobility particle sizer (SMPS, TSI, model: 3938, Shoreview, MN, USA).
The remaining jet engine oil fraction after the heating section was determined by
comparing the particle mass derived from the PNSD measurements at both
temperatures.
Impactor sampling and molecular characterisation. Detailed information on
sampling technique, sample preparation and extraction procedure can be found
elsewhere26. Briefly, we used a Micro Orifice Uniform Deposition Impactor (NanoMOUDI, Model 115, MSP, Minneapolis, MN, USA) at an air-quality monitoring
site in Frankfurt-Schwanheim and sampled particles on the three nano-stages
<56 nm. All stages were equipped with aluminium foils (TSI, diameter 47 mm and
thickness 0.015 mm), and the upper ten stages were coated with Apiezon® grease to
minimise the bounce-off of larger particles.
In the period of August to October 2019, we sampled UFPs for 18–54 h during
airport operating hours (5:00–23:00 CET) and during southerly wind direction.
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Fig. 4 Conceptual illustration of the UFP formation. Emissions of aircraft turbofan engines result in fast nucleation and growth of jet oil vapours in the near
field. Non-volatile emissions (nvPM) are not shown. Dimensions are not true to scale.
Without an active sampling airflow, we collected field blanks for 115 h on the three
nano-stages to estimate possible background concentrations regarding the target
compounds. We stored the filters until analysis at −20 °C. Due to the extensive
sampling time span, we assume that our UFP samples represent aircraft engines of
several types under various operating states. This is essential for covering the
average UFP emission of the whole airport and not of individual engines or certain
engine operating states.
We quantified the additives and jet oil esters using standard addition with
authentic and surrogate standards, respectively (Supplementary Figs. 7 and 8).
Targeted measurements of the jet engine oil constituents were carried out by using
ultra-high performance liquid chromatography (UHPLC)/heated electrospray
ionisation (HESI) coupled to an Orbitrap high-resolution mass spectrometer
(HRMS). Chromatographic separation of the jet engine oil constituents was
accomplished using a C18-reversed phase column (Details see Supplementary
Note 3). Using the standard addition method, we quantified 23 compounds in 25
ambient filter samples including 3 blank samples (Details see Supplementary
Note 4). Most of these compounds belong to the group of pentaerythritol- or
trimethylolpropane esters, which are utilised as jet engine oil base stocks
(Supplementary Table 2). Finally, we determined the jet engine oil mass of the
deposited UFPs after field blank correction.
Experimental loss determination in the Nano-MOUDI. Since sampling UFPs
with a Nano-MOUDI is accompanied by particle losses, we determined a loss
factor for each Nano-MOUDI stage (Details see Supplementary Note 5, Supplementary Figs. 9–11, and Supplementary Table 3). The loss of particles with an
aerodynamic diameter between 32–56 nm is 28% and for 18–32 nm particles 40%,
respectively. We were not able to experimentally determine a loss factor for the
smallest size bin of 10–18 nm particles, due to insufficient deposited mass. We
calculated the loss under the assumption that particle diffusivity is the main driving
force for sampling losses of extremely low-volatile compounds in the UFP size
range. We determined the dependency between particle diameter and diffusion
coefficient at 17.2 kPa and 20 °C (sampling condition of the 18–32 nm stage). To fit
the experimentally determined particle losses of the two larger stages, we applied a
damping term on the diffusion coefficient equation (Supplementary Fig. 11). Based
on the experimentally determined losses of the two upper nano-stages, we calculated a loss of ~58% for the smallest stage. This loss factor can be considered as a
conservative estimate, as it is only based on particle diffusive losses and not
including losses due to evaporation after impaction (see main text). By implementing these loss factors, we corrected the quantified jet oil filter mass and
determined the mass fraction of jet engine lubrication oils in airport-related UFPs.
residence time and particle losses in the inlet system. The PNSD was measured in
the size range of 10–500 nm at a temporal resolution of 5 min. Particle losses due to
sedimentation, inertial impaction and diffusion have been calculated and corrected
accordingly62. The UFP mass was determined by integration assuming spherical
particles. We calculated the particle mass for each filter collection interval exclusively during airport operating hours by converting the PNSD into a volume
distribution averaged over the sampling period using a unit density of 1 g cm−3 and
the Nano-MOUDI sampling flow rate of 0.6 m3 h−1. The particle density was
chosen according to the analysed jet engine oil densities (see safety data sheets) and
aircraft turbine engine studies50. Consequently, conversion of the measured
mobility diameter to aerodynamic diameter is not necessary63. We analysed the
SMPS data of seven filter sampling periods as no data is available for one sampling
period due to an instrument failure.
Volatility and saturation ratio of jet oil esters. The volatility of compounds
strongly determines their gas-to-particle partitioning behaviour. Hence, evaluation
of the jet oil base stocks using semi-empirical group contribution methods (SIMPOL.1 model55) and the volatility basis set (VBS52,53) enables the grouping of the
single ester compounds to volatility classes (ULVOC: ultra-low volatility-, ELVOC:
extremely-low volatility-, LVOC: low-volatility organic compound). Compound
classification is based on their volatility expressed as the logarithm of the saturation
mass concentration (log10 C i ), where the volatility is differentiated by one decade
in Ci , which is also assumed as uncertainty. The saturation mass concentration (Ci
(µg m−3)) is calculated as the inverse of the gas-to-particle phase partitioning
constant (K p )52 taking into account the weight fraction of the absorbing organic
material (om) phase (f om ), its average molecular weight (MW om , g mol−1), and the
activity coefficient (ζ i ) and vapour pressure (p0L;i , Torr)52,64 of compound i:
1
f om 760 R T
¼ Kp ¼
ζ i C i
MW om ζ i p0L;i 106
We calculated K p assuming the absorbing organic phase consists only of the
respective substance ðf om ¼ 1Þ, which leads to an ideal absorption affinity of the
molecules passing from the gas phase to the particle phase ðζ i ¼ 1Þ. The
compound’s affinity to the particle phase inversely correlates with ζ i 65. R is the gas
constant (8.2 × 10−5 m3 atm mol−1 K−1) and T (K) the temperature.
We converted the quantified base stock ester concentrations in the particle
phase (mVi (g m−3)) to gas phase number concentrations (cvi (m−3)) using the ideal
gas law (mi : quantified ester mass; M i : molecular mass in g mol−1):
cvi ¼
Ambient SMPS measurements. The PNSD at the sampling site was determined
using a SMPS including an electrostatic classifier (TSI, model: 3082), a Differential
Mobility Analyser (DMA, TSI, model: 3081) and a Condensation Particle Counter
(CPC, TSI, model: 3772). Ambient air was sampled through a stainless-steel tube
(inner diameter: 20 mm, length 1.6 m), using a PM2.5 inlet head at a flow rate of
1 m3 h−1. Prior entering the SMPS, the aerosol passes a Nafion dryer (1.2 m length,
flow rate of 0.3 m3 h−1) to stabilise the relative humidity below 40%. The actual
sample flow of the SMPS was 1 L min-1, the additional bypass is used to minimise
6
ð1Þ
mi N A
V Mi
ð2Þ
To determine whether jet oil constituents reach gas-phase supersaturation, we
calculated their gas-phase saturation ratio ðSi Þ66:
Si ¼
cvi M i 106
C i N A
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Content courtesy of Springer Nature, terms of use apply. Rights reserved
ð3Þ
COMMUNICATIONS EARTH & ENVIRONMENT | https://doi.org/10.1038/s43247-022-00653-w
Accounting for the temperature dependence of the saturation vapour pressure,
Ci can be described according to the Clausius-Clapeyron equation:
!
vap
ΔH i
1
1
ð4Þ
log10 C i ðT Þ ¼ log10 C i ð293:15 KÞ þ
293:15 K T
R lnð10Þ
where R = 8.314 × 10−3 kJ K−1 mol−1. The evaporation enthalpy ΔH i
can be approximated by:
vap
(kJ mol−1)
ð5Þ
ΔH i ¼ 11 log10 Ci ð293:15 KÞ þ 129
Despite the large uncertainties of this approach67, it still can be used to describe
a simple estimate of the temperature dependence of the oil partitioning. Finally, we
combined [Eq. 3] and [Eq. 4] to calculate the approximate temperature at which
the jet oil esters reach gas-phase supersaturation ðSi ¼ 1Þ in a cooling engine
exhaust plume [Eq. 6].
vap
T iS
¼1
¼
1
log10 ðN
cvi M i 106
Þ
A C i ð293:15 KÞ
lnð10Þ
1
R ΔH
293:15
vap
K
ð6Þ
i
Reporting summary. Further information on research design is available in the Nature
Portfolio Reporting Summary linked to this article.
Data availability
The data shown in this study is available at https://doi.org/10.5281/zenodo.6876277.
Received: 22 October 2022; Accepted: 1 December 2022;
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Acknowledgements
We thank Anett Dietze of the Leibniz Institute for Tropospheric Research (TROPOS) for
filter preparation and weighing. This research has been supported by the Deutsche
Forschungsgemeinschaft (DFG; German Research Foundation) (grant no. 410009325
and 428312742 (TRR 301)).
Author contributions
F.U. wrote the paper; designed research, performed the field sampling, sample preparation, measurements, lab experiments and majority of data analysis; L.C. performed
lab experiments and analysed data; F.D. and D.R. performed SMPS measurements; M.S.,
D.v.P., S.J., D.K., A.K. and J.C. advised on data interpretation; A.L.V. designed research,
advised on data analysis, data interpretation and manuscript writing; edited and revised
the manuscript; and directed the project administration. All authors commented on the
manuscript and contributed to the scientific discussion.
Funding
Open Access funding enabled and organized by Projekt DEAL.
Competing interests
The authors declare no competing interests.
Additional information
Supplementary information The online version contains supplementary material
available at https://doi.org/10.1038/s43247-022-00653-w.
Correspondence and requests for materials should be addressed to Alexander L. Vogel.
Peer review information Communications Earth & Environment thanks the anonymous
reviewers for their contribution to the peer review of this work. Primary Handling
Editors: Clare Davis. The peer reviewer reports are only from the second round of
peer-review.
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